Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-lcc!well!msudoc!crlt!michael From: michael@crlt.UUCP (Michael McClary) Newsgroups: comp.graphics Subject: Re: Colour perception Message-ID: <671@crlt.UUCP> Date: Sun, 15-Mar-87 20:59:21 EST Article-I.D.: crlt.671 Posted: Sun Mar 15 20:59:21 1987 Date-Received: Tue, 17-Mar-87 01:21:50 EST References: <8703121101.AA09386@ingres.Berkeley.EDU> Organization: CRLT , Ann Arbor, MI Lines: 74 Summary: A couple minor points. In article <505@ubu.warwick.UUCP> rolf@warwick.UUCP (Rolf Howarth) writes: >I thought I'd ask if anyone could explain to me how the human brain >perceives colour. >red + green light "gives you yellow", where "yellow" is also what you see >at a particular position in the spectrum (when you shine white light >through a prism). As far as I understand it, these two "yellows" are different >spectroscopically , yet the eye perceives them to be the same colour. In article <8703121101.AA09386@ingres.Berkeley.EDU>, hatcher@INGRES.BERKELEY.EDU (Doug Merritt) replies with a lengthly discussion of the mechanism of color perception. The main point is that the color sensation is derived from data collected by three groups of narrow-band light-sensing cells in the retina, called "cones", which have peaks of spectral sensitivity in the red, blue, and green. It might be possible to draw incorrect answers to Rolf's questions from Doug's entry, however. (I have only Doug's reply available, unfortunately - we've been having trouble with our news software lately. Please exscuse me if I'm off in the ozone because I made a bad inferrence about what Rolf asked.) Pure colors of different frequencies stimulate the three types of cones in amounts that are a function of the product of their intensity and how much they are absorbed by a sensitive chemical (called a "pigment") in the cone. Different types of cones have different pigments with different curves of light-absorption versus frequency. The sensitivity curves of the various pigments overlap, preventing "holes" in the response. Thus green light stimulates the green cones a lot and the blue and red cones very little, while yellow light stimulates both the blue and green conse considerably and the red cones very little. The brain 'thinks' "Buncha blue, buncha green, tiny bit of red. Aha! It's yellow!" For most colors of light (which are mixes of various colors already), you can approximate the result in the cones by mixing appropriate amounts of three colors of light, tuned to the responses of the three types of cones. For very pure spectral colors located between the various spectral peaks of the cone responses, you can't quite do it, because the approximating colors stimulate the third receptor more than the pure spectral light you're trying to model does. (I believe this is most apparent with yellow, and is why you can't get a very good pure yellow from a color TV.) There is an additional effect, caused by the approximation not quite stimulating the rods (the very sensitive black-and-white broadband receptors) correctly, but this is minor because the visual processing gives priority to the cones when light levels are sufficient to operate them. As to color-blind people, color-blindness comes in many forms. Some are just missing one or more visual pigment. The remaining color pigments are stimulated in the same way as their counterparts in the normally-sighted, so the TV image they see is about as good a match as that seen by the normally-sighted. (They might have more participation from the rods in the color sensation, however, which could distort things a little bit.) In others, one or more of the cones have a mutated pigment - either another copy of one from a different type of cone, or one which has an abnormal sensitivity curve. If they have, say, a copy of the green pigment in the blue receptors, they still get about the same image off a TV as they get from the real-world. If they have something with a sensitivity peak far from TV's standard colors, though, (a yellow-sensitve pigment in the blue cones, for instance) they get serious color distortion. It would probably be possible to design a special television system for these people, with phospors and camera color filters that match their eyes, but it would be dreadfully expensive to do so, and would yeild similarly bad color distortion when viewed by the rest of us. "I've got code in my node." | UUCP: ...!ihnp4!itivax!node!michael | AUDIO: (313) 973-8787 Michael McClary | SNAIL: 2091 Chalmers, Ann Arbor MI 48104 Above opinions are the official position of McClary Associates. Customers may have opinions of their own, which are given all the attention paid for.